Выражение встречается в следующем контексте:

Condition of sooting nozzles and cleaning of tubes must be checked at regularly through the maintenance doors. The best cleaning results are achieved and damage to the exhaust gas boiler caused by corrosion are avoided by experience, when, during cleaning, hot water is circulated in tubes and the circulation is continued until the whole exhaust gas boiler is dry (does not concern the natural circulation boilers).
Заранее спасибо